The invention relates to an electric machine having a rotor which carries permanent magnets, and more precisely, a rotor in which the magnets are retained within recesses of the rotor. The electric machines in question are commonly described as having xe2x80x9cembedded magnets.xe2x80x9d This rotor arrangement principle is widely applied for automatic synchronous machines with flux concentration.
The size of a rotating electric machine of this type depends on its rated load torque. The greater the torque that a motor is capable of delivering, the more voluminous the electric motor, everything else being equal. There are, however, applications for which it is desirable to attain both high power and great compactness of the motor. To give simply one concrete example, when it is sought to install electric traction motors in the wheels of motor vehicles, it is desirable to be able to develop power equal to at least 10 kW per motor and even, more often, at least 25 or 30 kW per motor for as low a weight as possible in order not to overload unsprung weights. It is also desirable for the space occupied to be very compact, exceeding the interior volume of the wheel as little as possible in order not to interfere with the clearances of the parts of the vehicle in suspension movements and with other types of motion of the wheel relative to the body of the vehicle.
Those two imperatives (high power and reduced weight and space) make highly problematical the installation of electric traction motors in the wheels of passenger vehicles, except to radically improve the weight/power ratio of the electric machines currently available on the market.
The choice of a high speed for an electric motor is a solution which makes possible, at given power, to reduce the torque and therefore the space required. In other words, for a given motor rating, the greater its rated speed of rotation, the less the space it requires. But raising the speed of rotation of a rotating electric machine poses numerous problems of mechanical performance that are especially difficult to solve, if it is desired to limit the weight and the space occupied by said rotating electric machine as much as possible.
The object of the invention is to propose a rotating electric machine design which makes it possible to attain high speeds of rotation, at least up to 12,000 revolutions per minute, for example, without encountering any problem either of torque transmission or of rotor centrifugation.
According to the invention, the rotating electric machine, having an outer casing forming a support structure for a stator, has a rotor comprising:
a shaft made in a single piece of nonmagnetic material mounted by means of bearings on the outer casing, said bearings defining an axis of rotation of said shaft;
an assembly embodying a plurality of pole pieces and permanent magnets surrounding the shaft, the pole pieces defining between them chambers extending from the surface of the shaft to an air gap between rotor and stator, said chambers containing said permanent magnets;
a lateral flange mounted axially at each end of said shaft;
tie rods connecting the lateral flanges passing through each pole piece and enclosing each pole piece between the lateral flanges, in which said shaft, seen in section perpendicular to the axis of rotation, forms a convex noncircular figure, cooperating with said assembly to immobilize it in relative rotation on said shaft.
The configuration of the shaft makes possible a torque transmission direct from the pole pieces to the shaft, at least in localized fashion or preferably over the whole axial length of the pole pieces. The shaft, seen in section perpendicular to the axis of rotation, preferably forms a regular convex polygon, containing plane facets separated by edges. Each pole piece, seen in section perpendicular to the axis of rotation, then presents generally radial edges opposite the magnets and, opposite the shaft, forms a reenterant angle designed to center said pole piece on one of the edges of the shaft. A hexagonal polygon is particularly favorable to torque passage from the pole pieces to the shaft, while ensuring good compactness of the shaft.